Process for the manufacture of high alpha-cellulose fiber



Patented Sept. 27, 1927.

UNITED STATES 1,643,355 PATENT OFFICE.

GEORGE A. RICHTER AND MILTON O. SCHUR, OF BERLIN, NEW HAMPSHIRE, AS-

SIGNORS TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION PROCESS FOR THE-MANUFACTURE OF HIGH ALPHA-CELLULOSE FIBER.

No Drawing. Application filed April 9, 1925, Serial No. 21,963 Renewed November 3, 1926.

This invention relates to the preparation of high alpha-cellulose fiber. High alphacellulose fiber may be produced by cooking unbleached sulphite pulp with an alkaline 6 cooking liquor (a caustic soda solution being especially adapted for this purpose), and then bleaching the cooked product. This cooking may take place at any suitable temerature and pressure, although we have 10 ound it to be more economical and expedient to cook in open tanks under atmospheric pressure. The alkali in solution reacts with and dissolves the lignin and the less resistant beta and gamma celluloses present in the pulp, so that when the liquor is drawn ofi after the' necessary period of digestion the resultant product is easy to bleach and contains at least 94% alpha-cellulose ,fiber. High alpha-cellulose fiber may be employed 'as a substitute for cotton fiber in paper making and in the preparation of the cellulose esters, the cellulose ethers, and in the preparation of other compounds which require the use of substantially pure cellulose.

A drawback to the use of caustic soda alone as the alkaline digesting agent is, however, that it renders the process expensive.

The object of this invention is to provide a more economical method for the prepara-.

tion of a hi h alpha-cellulose fiber, yielding a product w ich will possess the same characteristics as that produced by the use of caustic soda. Briefly, this is attained by cooking the pulp in slaked lime cooking liquor and in caustic soda liquor. Other advantages which may be obtained in this invention will subsequently be pointed out.

In a co-pending companion application filed of even date herewith, Serial No. 21,962,

which has become Patent No. 1,599,489, the use of milk of lime has been described as bein efi'ective in removing beta and gamma cellu ose from unbleached sulphite pulp but it is pointed out that the caustic soda cooking liquor is much more effective in removing the coioring matters therefrom. In order to produce fiber of high whiteness, these colormg substances must be removed. -In that application we have ointed out that in order to sufiiciently whiten the pulp after an ordinary lime-dlgestion, it is necessary to use an excess of bleach, thereby resulting in a weakening of the fiber and a reduction of the alpha-cellulose content.

We have found that unbleached sulphite pulp may be cooked with a mixture of slaked lime (Ca(OH) and caustic soda (NaOI-I), the proportions of these re-agents and the method of using them being so chosen that the beta and gamma celluloses and the lignin and other coloring matter are dissolved and thereby removed therefrom. The cooked product ,upon mild bleaching is very white and contains over 94% alpha cellulose.

While a mixed caustic and lime cooking liquor may be employed, the pulp may be cooked first in caustic sodaliquorand the lime added later thereto, or it-may be cooked in a lime liquor first and the caustic addedlater,-the last procedure being preferred, for in this case the lime reacts with the less resistant beta and gamma celluloses first, and the caustic soda added later'is consumed substantially only in the removal of the lignin and other coloring matter. If it were I added otherwise it might also be consumed in reacting with the less resistant celluloses rather than with the coloring substances. Although a gain in bleachability of the product may be secured by Washing the stock between the alkaline treatments, for reasons of economy in steam and equipment it is preferred to add the latter alkali directly after the first alkali treatment has been completed. The method may be practiced about as follows I A 10% suspension of unbleached sulphite pulp is mixed thoroughly with milk of lime containing about 6% to 8% active calcium, oxide based upon the bone-dry fiber weight. The mixture is brought to about 200 to 210 F. in an open tank by means of live steam, and is thoroughly agitated while being maintained at this temperature for from five to eight hours, depending upon the characteristics of the particular sulphite stock undergoing treatment. The beta and gamma celluloses react with and are efi'ectively removed by the lime. Then 1% to 2% caustic soda based upon the bone-dry fiber is 1 added preferably in the form of a boilin hot 1% to 5% solution, and is thoroug y mixed with the pulp. About an hour after the addition of the caustic soda,-the lignin and other coloring matters have reacted suf-v ficiently therewith, and the stock is then washed clear of reaction roducts. From 4% to 6% of bleach (OaO 1,), based on the 7 process, in which the pulp is treated with a the unbleached sulphite chlorine solution.

It may be advantageous to combine a chlorine solution pre-treatment which removes or renders reactive a portion of the lignin in the sulphite pulp prior to the practice of the previously described process, so that economy in the use of chemicals may further be increased with the easier attainment of a very white alpha-cellulose product. This .may be done by treating the unbleached sulphite pulp prior to the alkaline digestion a chlorine solution, a portion of the lignin and other coloring substances reacting with and being oxidized by the chlorine. Such a pre-treatment would make possible the use of less caustic in the subsequent alkaline cook.

Another important advantage may be obtained by the use of the chlorine pre-treatment process, which is that the wood chips may be slightly undercooked in the sulphite digesters, thus producing a longer fiber. The pulp thus obtained would ordinarily be more difficult to bleach owing to the fact that it contains a higher percent of lignin, but the quantity. of oxidizing agents used in the pre-treatment may be increased so as to remove or render more reactive the additlonal amount of li in. The pre-treated pulp will then be sub ected to the caustic and lime cooking solution and upon bleaching w1ll yield a very white alpha-cellulose fiber of the desired chemical and physical characteristics.

It is, of course, possible to replace all or a llpart of the caustic soda by other strongl a aline substances which readily react with the lignin and other coloring substances in ul such, for example, as sodium sulphidb FNB S) or sodium'cpolysulphide (Na,S,) or caustic potash .(K H) and dolomite lime, containing magnesium oxide, may be used in place of all or part of the calcite lime. Furthermore, it 1s not necessary that the processes be carried out in 0 en tanks. i Closed digesters may be used an the alkaline treatment carried outunder higher temperature and pressure conditions, in which case the caustic solution in, the foregoing example would be introduced "into the digester by a pump, injector, or

other suitable means. It is to be emphasized, on the other hand, as in the copending application, Serial No. 21,962, which has become Patent No. 1,599,489, that the use of high temperature and pressure would complicate the process and call for the use of a comparatively intricate apparatus, all of which tends to make the process as or more expensive than a straight caustic soda treatment.

Since lime is much. cheaper than an equiv- I atmospheric pressure, and finally bleaching the product, as this forms the subject-matter of our application Serial No. 716,154, filed Ma 27,1924.

at we claim is: 1. The

cellulose ber, which comprises cooking un- I bleached sulphite pulp in a solution of an alkaline compound of an alkali metal, containing in suspension an alkaline earth metal hydroxide.

2. The rocess of producing a high alphacellulose, ber, which comprises cooking unbleached sulphite pulp in a suspension of an alkaline earth metal hydroxide, adding a solution of an alkaline compound of an alkali metal, and continuing the cookin 3. The process of producing a high alphacellulose fiber, which comprises cooking unbleached sulphite pulp at atmos heric pressure in a suspension oflime, adding a solution of sodium hydroxide, and continuing the cooking. k p

4. The rocess of producing a high alpha-' cellulose ber, which comprises treating unbleached sulphite pulp with a solution of chlorine, insuflieigent "in quantity to cause complete bleaching, boilin with a suspension of lime, adding a solution of sodium hydroxide and continuing the boiling.

5. The rocess of producing a high alphacellulose ber, which comprises treating unbleached sulphite pulp with a solution of chlorine, insuflicient in quantity to cause complete bleaching, separating the pulp. from soluble reaction-products, boiling it with a suspension of lime, adding a solution of sodium hydroxide, continuing-the boiling, and again so arating the pulp from soluble reaction pro ucts.

a bleached 6. The process of roducing high alpha-cellulose her, which comprises cooking unbleached sulphite pulp with a' sus- 1process of producing a high alphapension of lime, adding a solution of sodium hydroxide, continuing the cooking, separating the pulp from soluble reaction-products, and bleaching it with calcium hypochlorite solution.

7. The process of producing a superbleached high alpha-cellulose fiber which comprises cooking unbleached sulphite pulp with a suspension of lime, adding a solution of sodium hydroxide, continuing the cooking, separating the pulp from soluble reaction products, bleaching it with calcium hypochlorite solution and superbleaching it with a chlorine solution.

8. The process of producing a high alphacellulosepulp, which comprises cooking unbleached sulphite pulp in a lime suspension containing about 6% to 8% calcium oxide based on the weight of bone-dry fiber and in a caustic soda solution containing 1% to 2% sodium h droxide based on the weight of bone-dry fi er.

9. The process of producingia high alphacellulose pulp, which comprises undercooking chips in a sulphite digesting liquor, treating the product obtained from said sulphite digesters with chlorine to eliminate a portion of the lignin but without causing a complete bleaching reaction, and cookln said treated product with agitation in a mil of lime suspension to remove beta and gamma celluloses therefrom, adding a caustic soda solution to the mass and continuing the cooking to remove more ligneous matter therefrom.

In testimony whereof we have aflixed our signatures.

GEORGE A. RICHTER. LHLTON O. SCHUR.- 

